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.TH MQ_OPEN 3C "Feb 5, 2008"
.SH NAME
mq_open \- open a message queue
.SH SYNOPSIS
.LP
.nf
#include <mqueue.h>

\fBmqd_t\fR \fBmq_open\fR(\fBconst char *\fR\fIname\fR, \fBint\fR \fIoflag\fR,
     \fB/* unsigned long\fR \fImode\fR, \fBmq_attr\fR \fIattr\fR */  ...);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBmq_open()\fR function establishes the connection between a process and a
message queue with a message queue descriptor. It creates a open message queue
description that refers to the message queue, and a message queue descriptor
that refers to that open message queue description. The message queue
descriptor is used by other functions to refer to that message queue.
.sp
.LP
The \fIname\fR argument points to a string naming a message queue. The
\fIname\fR argument must conform to the construction rules for a  path-name. If
\fIname\fR is not the name of an existing message queue and its creation is not
requested, \fBmq_open()\fR fails and returns an error. The first character of
\fIname\fR must be a slash  (\fB/\fR) character and the remaining characters of
\fIname\fR cannot include any slash characters.  For maximum portability,
\fIname\fR should include no more than 14 characters, but this limit is not
enforced.
.sp
.LP
The \fIoflag\fR argument requests the desired receive and/or send access to the
message queue. The requested access permission to receive messages or send
messages is granted if the calling process would be granted read or write
access, respectively, to a file with the equivalent permissions.
.sp
.LP
The value of \fIoflag\fR is the bitwise inclusive \fBOR\fR of values from the
following list. Applications must specify exactly one of the first three values
(access modes) below in the value of \fIoflag\fR:
.sp
.ne 2
.na
\fB\fBO_RDONLY\fR \fR
.ad
.RS 13n
Open the message queue for receiving messages. The process can use the returned
message queue descriptor with \fBmq_receive\fR(3C), but not \fBmq_send\fR(3C).
A message queue may be open multiple times in the same or different processes
for receiving messages.
.RE

.sp
.ne 2
.na
\fB\fBO_WRONLY\fR \fR
.ad
.RS 13n
Open the queue for sending messages. The process can use the returned message
queue descriptor with \fBmq_send\fR(3C) but not \fBmq_receive\fR(3C). A message
queue may be open multiple times in the same or different processes for sending
messages.
.RE

.sp
.ne 2
.na
\fB\fBO_RDWR\fR \fR
.ad
.RS 13n
Open the queue for both receiving and sending messages. The process can use any
of the functions allowed for \fBO_RDONLY\fR and \fBO_WRONLY\fR. A message queue
may be open multiple times in the same or different processes for sending
messages.
.RE

.sp
.LP
Any combination of the remaining flags may additionally be specified in the
value of \fIoflag\fR:
.sp
.ne 2
.na
\fB\fBO_CREAT\fR \fR
.ad
.RS 15n
This option is used to create a message queue, and it requires two additional
arguments: \fImode\fR, which is of type \fBmode_t\fR, and  \fIattr\fR, which is
pointer to a \fBmq_attr\fR structure. If the pathname, \fIname\fR, has already
been used to create a message queue that still exists, then this flag has no
effect, except as noted under \fBO_EXCL\fR (see below). Otherwise, a message
queue is created without any messages in it.
.sp
The user ID of the message queue is set to the effective user ID of process,
and the group ID of the message queue is set to the effective group ID of the
process. The file permission bits are set to the value of \fImode\fR, and
modified by clearing all bits set in the file mode creation mask of the process
(see \fBumask\fR(2)).
.sp
If \fIattr\fR is non-\fINULL\fR and the calling process has the appropriate
privilege on \fIname\fR, the message queue \fImq_maxmsg\fR and \fImq_msgsize\fR
attributes are set to the values of the corresponding members in the
\fBmq_attr\fR structure referred to by \fIattr\fR. If \fIattr\fR is
non-\fINULL\fR, but the calling process does not have the appropriate privilege
on \fIname\fR, the \fBmq_open()\fR function fails and returns an error without
creating the message queue.
.RE

.sp
.ne 2
.na
\fB\fBO_EXCL\fR \fR
.ad
.RS 15n
If both \fBO_EXCL\fR and \fBO_CREAT\fR are set, \fBmq_open()\fR will fail if
the message queue \fIname\fR exists. The check for the existence of the message
queue and the creation of the message queue if it does not exist are atomic
with respect to other processes executing \fBmq_open()\fR naming the same
\fIname\fR with both \fBO_EXCL\fR and \fBO_CREAT\fR set.  If \fBO_EXCL\fR and
\fBO_CREAT\fR are not set, the result is undefined.
.RE

.sp
.ne 2
.na
\fB\fBO_NONBLOCK\fR \fR
.ad
.RS 15n
The setting of this flag is associated with the open message queue description
and determines whether a \fBmq_send\fR(3C) or \fBmq_receive\fR(3C) waits for
resources or messages that are not currently available, or fails with
\fBerrno\fR set to \fBEAGAIN\fR. See \fBmq_send\fR(3C) and \fBmq_receive\fR(3C)
for details.
.RE

.SH RETURN VALUES
.sp
.LP
Upon successful completion, \fBmq_open()\fR returns a message queue descriptor;
otherwise the function returns \fB(mqd_t)\(mi1\fR and sets \fBerrno\fR to
indicate the error condition.
.SH ERRORS
.sp
.LP
The  \fBmq_open()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEACCES\fR \fR
.ad
.RS 17n
The message queue exists and the permissions specified by \fIoflag\fR are
denied, or the message queue does not exist and permission to create the
message queue is denied.
.RE

.sp
.ne 2
.na
\fB\fBEEXIST\fR \fR
.ad
.RS 17n
\fBO_CREAT\fR and \fBO_EXCL\fR are set and the named message queue already
exists.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR \fR
.ad
.RS 17n
The \fBmq_open()\fR operation was interrupted by a signal.
.RE

.sp
.ne 2
.na
\fB\fBEINVAL\fR \fR
.ad
.RS 17n
The \fBmq_open()\fR operation is not supported for the given name, or
\fBO_CREAT\fR was specified in \fIoflag\fR, the value of \fIattr\fR is not
\fINULL,\fR and either \fBmq_maxmsg\fR or \fBmq_msgsize\fR was less than or
equal to zero.
.RE

.sp
.ne 2
.na
\fB\fBEMFILE\fR \fR
.ad
.RS 17n
The number of open message queue descriptors in this process exceeds
\fBMQ_OPEN_MAX,\fR of the number of open file descriptors in this process
exceeds \fBOPEN_MAX.\fR
.RE

.sp
.ne 2
.na
\fB\fBENAMETOOLONG\fR \fR
.ad
.RS 17n
The length of the \fIname\fR string exceeds \fBPATH_MAX,\fR or a pathname
component is longer than \fINAME_MAX\fR while \fB_POSIX_NO_TRUNC\fR is in
effect.
.RE

.sp
.ne 2
.na
\fB\fBENFILE\fR \fR
.ad
.RS 17n
Too many message queues are currently open in the system.
.RE

.sp
.ne 2
.na
\fB\fBENOENT\fR \fR
.ad
.RS 17n
\fBO_CREAT\fR is not set and the named message queue does not exist.
.RE

.sp
.ne 2
.na
\fB\fBENOSPC\fR \fR
.ad
.RS 17n
There is insufficient space for the creation of the new message queue.
.RE

.sp
.ne 2
.na
\fB\fBENOSYS\fR \fR
.ad
.RS 17n
The \fBmq_open()\fR function is not supported by the system.
.RE

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Committed
_
MT-Level	MT-Safe
_
Standard	See \fBstandards\fR(7).
.TE

.SH SEE ALSO
.sp
.LP
.BR exec (2),
.BR exit (2),
.BR umask (2),
.BR mq_close (3C),
.BR mq_receive (3C),
.BR mq_send (3C),
.BR mq_setattr (3C),
.BR mq_unlink (3C),
.BR sysconf (3C),
.BR mqueue.h (3HEAD),
.BR attributes (7),
.BR standards (7)
.SH NOTES
.sp
.LP
Due to the manner in which message queues are implemented, they should not be
considered secure and should not be used in security-sensitive applications.
.sp
.LP
Solaris 2.6 was the first release to support the Asynchronous Input and Output
option. Prior to this release, this function always returned \fB\(mi1\fR and
set \fBerrno\fR to \fBENOSYS\fR.
